First, observe a stranger out in public (not necessarily at night). Jot down the details of the person’s appearance and dress. How does that person gesture or move? If you hear the person talking, how does he or she speak?
Next, use your imagination and ask yourself the following questions. Where was this person an hour before you saw them? What was he or she doing then? What will this person do an hour from now? What is he or she thinking about as you watch them? What desires currently drive this person’s life? Write down these details and then write a general paragraph about the person.
Now observe another stranger and do the same thing described above for the first person.
Here’s the best part. Write a brief scene (250 to 500 words) in which these two characters interact with each other. You can decide if they already know each other or not.
Whether you write more than one scene is up to you, but if nothing else, this exercise will spark your creative impulses and give you practice in creating characters.